1. Field of the Invention
This invention relates to well logging methods and apparatus and more particularly to nuclear well logging techniques to determine the presence of undesired water flow in cement voids or channels behind steel well casing in a cased well borehole as well as flow in the borehole and adjacent tubing.
2. Description of the Related Art
Undesired fluid communication along the cased portion of a well between producing zones has long been a problem in the petroleum industry. The communication of fresh or salt water from a nearby water sand into a petroleum production sand can contaminate the petroleum being produced by the well to an extent that production of petroleum from the well can become commercially unfeasible due to the “water cut”. Similarly, in near surface water wells used for production of fresh water for city or town drinking supplies or the like, the contamination of the fresh water drinking supply by the migration of salt water from nearby sands can also contaminate the drinking water supply to the extent where it is unfit for human consumption without elaborate contaminant removal processing.
In both of these instances, it has been found through experience over the course of years that the contamination of fresh water drinking supplies or producing petroleum sands can occur many times due to the undesired communication of water from nearby sands down the annulus between the steel casing used to support the walls of the borehole and the borehole wall itself. Usually steel casing which is used for this purpose is cemented in place. If a good primary cement job is obtained on well completion, there is no problem with fluid communication between producing zones. However, in some areas of the world where very loosely consolidated, highly permeable sands are typical in production of petroleum, the sands may later collapse in the vicinity of the borehole even if a good primary cement job is obtained. This can allow the migration of water along the outside of the cement sheath from a nearby water sand into the producing zone. Also, the problem of undesired fluid communication occurs when the primary cement job itself deteriorates due to the flow of fluids in its vicinity. Similarly, an otherwise good primary cement job may contain longitudinal channels or void spaces along its length which permit undesired fluid communication between nearby water sands and the producing zone.
Another problem which can lead to undesired fluid communication along the borehole between producing oil zones and nearby water sands is that of the so called “microannulus” between the casing and the cement. This phenomenon occurs because when the cement is being forced from the bottom of the casing string up into the annulus between the casing and the formations, (or through casing perforations), the casing is usually submitted to a high hydrostatic pressure differential in order to force the cement into the annulus. The high pressure differential can cause casing expansion. When this pressure is subsequently relieved for producing from the well, the previously expanded casing may contract away from the cement sheath formed about it in the annulus between the casing and the formations. This contraction can leave a void space between the casing and the cement sheath which is sometimes referred to as a microannulus. In some instances, if enough casing expansion has taken place during the process of primary cementing (such as in a deep well where a high hydrostatic pressure is required) the casing may contract away from the cement sheath leaving a microannulus sufficiently wide for fluid to communicate from nearby water sands along the microannulus into the producing perforations and thereby produce an undesirable water cut.
U.S. Pat. No. 4,032,780 to Paap et al. teaches a method of determination of the volume flow rate and linear flow velocity of undesired behind casing water flow is provided. A well tool having a 14 MeV neutron source is used to continuously irradiate earth formations behind well casing. The continuous neutron irradiation activates elemental O16 nuclei in the undesired water flow to be detected. Dual spaced gamma ray detectors located above or below the neutron source detect the decay of unstable isotope N16 and from these indications the linear flow velocity of the undesired water flow is deduced. By then estimating the distance R to the undesired flow region the volume flow rate V may be deduced.
U.S. Pat. No. 5,461,909 to Arnold teaches a modification of the Paap technique in which the linear flow velocity, the Full Width Half Maximum time period, and the total count are determined directly from the resulting count rate curve. The radial position and the flow rate are determined using the predetermined relationship between the Full Width Half Maximum time period, radial position, and linear flow velocity, and the predetermined relationship between linear flow velocity, radial position, and the ratio of the flow rate to the total count for the logging tool. The direction of flow is determined by sensing the presence or absence of flowing N16 above or below the source.
The references discussed above do not address the problem of more than one type of fluid flowing in the borehole. U.S. Pat. No. 5,404,752 to Chace et al. teaches a method for measuring the velocities of water volumes flowing co-directionally in separate conduits nested such as in injection or production well-bores. The method allows an oxygen activation measurement of the velocity of the water flow in the tubing-casing annulus in the presence of water flowing in the tubing string in the same direction. The method allows continuous logging at variable or constant cable velocities or stationary logging. Based on the method of velocity gauging, the method isolates the signal from the annular flow and can produce a continuous log of linear and volumetric annular flow rates with depth.
The methods discussed above are based on measurements of total counts within a specified energy window. As noted above, the method of Paap requires continuous irradiation. The method of Arnold uses a pulsed neutron source and requires correction for the background signal. In Arnold and in Chace, pulsing is carried out at relatively high frequencies. The method of Chace, when applied to dual flow, first determines an inner flow rate and then uses this determined inner flow rate for determination of an outer flow rate. It would be desirable to have a method in which such correction for background signals and the sequential determination of flow rates is not necessary. The present invention addresses this need.